1. Field of the Invention
The present invention relates generally to an operational transconductance amplifier (OTA) and more particularly, to a multi-output OTA.
2. Description of the Related Art
The OTA is an amplifier capable of controlling output current based on input voltage and can be equivalent to a resistor and an active component in the circuit. Besides, none of any resistor is needed in the circuit design and the equivalent resistance can be acquired by control of bias current.
As shown in FIG. 1, a conventional OTA 10 is adapted for connection with multiple input voltage sources VDD, Vinn, Vinp and includes a source degeneration unit 12, two current cancellation units 13 and 14, two current division units 15 and 16, and a differential output unit 18. The input voltage source VDD is to provide voltage supply. The two input voltage sources Vinn and Vinp are to provide differential input voltage.
The source degeneration unit 12 includes three transistors 122, 124, 126, which are operated in the triode region to simulate resistance. Sources S of these transistors 122, 124, 126 are interconnected for receiving the voltage of the input voltage source VDD. Gates G of these transistors 122, 124, 126 are interconnected. Each of the transistors 122 and 124 includes a node n1 or n2 at its drain D.
The current cancellation units 13 and 14 are connected with the two nodes n1 and n2 of the source degeneration unit 12 and the differential output unit 18. The two current cancellation units 13 and 14 can receive and convert the voltage of the input voltage sources Vinn and Vinp to generate an output current and the output current flows to output ends Voutp and Voutn of the differential output unit 18.
The two current division units 15 and 16 are connected with the two current cancellation units 13 and 14, respectively. The current division unit 16 is to convert the voltage of the input voltage sources Vinn and Vinp into current and conduct the current to the grounded ends to diminish the transconductance. Based on the technique of division, only a little output current is utilized, so the current utilization efficiency is lower. Besides, the two current division units 15 and 16 need larger layout area.
When small signal analysis is applied to the conventional OTA 10 shown in FIG. 1, it is known that all of small-signal electronic flows flow through the source degeneration unit 12 and a voltage stress ΔV is generated between the two nodes n1 and n2 of the source degeneration unit 12. In DC analysis, voltage stress ΔV of the source degeneration unit 12 is constant. When small-signal voltage is inputted, the voltage stress ΔV of the source degeneration unit 12 can be affected by the small-signal current iReq, so if the ratio between the respective small-signal currents (iReq=i132+i134+i152) generated by the two current cancellation units 13 and 14 and the two current division units 15 and 16 is relatively constant, such small-signal currents can be more linear. Since left and right transistors of the OTA 10 shown in FIG. 1 are symmetrical, in the process of the small-signal analysis, one of the left and right transistors of the OTA 10 is analyzed only where the transconductance of the transistor is equal to the specific value of the output current to the input voltage. After the small-signal analysis, the transconductances g132, g134, g152 of transistors 132, 134, 152 of the current cancellation unit 13 and the current division unit 15 can be acquired from the following equations.
            g      132        =                  1                  nV          T                    ⁢                                    (                          W              L                        )                    132                ·                  I          o                ·                              exp            ⁡                          (                                                V                  SG                                                  nV                  T                                            )                                ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                                            V                          S                                                -                                                  V                          outn                                                                                            V                        T                                                                              )                                                      ]                                          g      134        =                  1                  nV          T                    ⁢                                    (                          W              L                        )                    134                ·                  I          o                ·                              exp            ⁡                          (                                                V                  SG                                                  nV                  T                                            )                                ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                                            V                          S                                                -                                                  V                          outp                                                                                            V                        T                                                                              )                                                      ]                                          g      152        =                  1                  nV          T                    ⁢                                    (                          W              L                        )                    152                ·                  I          o                ·                              exp            ⁡                          (                                                V                  SG                                                  nV                  T                                            )                                ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                                            V                          S                                                -                        0                                                                    V                        T                                                                              )                                                      ]                                                  g        152            :                        g          134                :                  g          132                      =                                        (                          W              L                        )                    152                ⁡                  [                      1            -                          exp              ⁡                              (                                  -                                                                                    V                        S                                            -                      0                                                              V                      T                                                                      )                                              ]                    :                                                  (                              W                L                            )                        134                    ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                                            V                          S                                                -                                                  V                          outp                                                                                            V                        T                                                                              )                                                      ]                          :                                            (                              W                L                            )                        132                    ⁡                      [                          1              -                              exp                ⁡                                  (                                      -                                                                                            V                          S                                                -                                                  V                          outn                                                                                            V                        T                                                                              )                                                      ]                              
where W denotes the physical width of the transistor, and L denotes the physical length of the transistor.
The two current division units 15 and 16 are directly grounded to make the transconductance thereof be greatly affected by voltage of source end Vs as indicated in the aforesaid equation g152. In other words, if the input voltage VDD is changed, the transconductance of g152 will be directly affected, indicating that the transconductance of the conventional OTA 10 is not only affected by the physical dimension of the transistor but the variation of the input voltage.
As illustrated in FIG. 2, to realize a third-order low-pass differential transconductance capacitive filter 40, seven conventional OTAs 401-407 and three capacitors 421-423 are needed.